Semiconductor nanoparticles, method of producing semiconductor nanoparticles, and light-emitting device

ABSTRACT

Provided is a ternary or quaternary semiconductor nanoparticle that enables the band-edge emission and a less toxic composition. A semiconductor nanoparticle is provided that contains Ag, In, and S and has an average particle size of 50 nm or less, wherein the ratio of the number of atoms of Ag to the total number of atoms of Ag and In is 0.320 or more and 0.385 or less, the ratio of the number of atoms of S to the total number of atoms of Ag and In is 1.20 or more and 1.45 or less. The semiconductor nanoparticle is adapted to emit photoluminescence having a photoluminescence lifetime of 200 ns or less upon being irradiated with light having a wavelength in a range of 350 nm to 500 nm.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Japanese Patent Application Nos.2016-173446, filed on Sep. 6, 2016, and 2017-034613, filed on Feb. 27,2017. The entire disclosures of Japanese Patent Application Nos.2016-173446 and 2017-034613 are hereby incorporated by reference.

BACKGROUND Technical Field

The present disclosure relates to semiconductor nanoparticles, a methodof producing the semiconductor nanoparticles, and a light-emittingdevice using the semiconductor nanoparticles.

Description of Related Art

White light-emitting devices, which are used as backlights of liquidcrystal display devices and the like and which utilize photoluminescenceemission from quantum dots (also called “semiconductor quantum dots”),have been proposed. Fine particles of a semiconductor with a particlesize of 10 nm or less, for example, are known to exhibit a quantum sizeeffect. Such nanoparticles are called the quantum dots. The quantum sizeeffect is a phenomenon where a valence band and a conduction band, eachof which is regarded as continuous in bulk particles, become discretewhen the particle size is on the nanoscale, whereby a bandgap energy isvaried in accordance with their particle size.

Quantum dots absorb light and emit light corresponding to the bandgapenergy thereof. Thus, quantum dots can be employed as a wavelengthconversion material in light-emitting devices. Light-emitting devicesusing quantum dots are proposed, for example, in Japanese UnexaminedPatent Application Publication No. 2012-212862 and Japanese UnexaminedPatent Application Publication No. 2010-177656. More specifically, aportion of the light emitted from a light-emitting diode (LED) chip isabsorbed by quantum dots, and photoluminescence of another color isemitted from the quantum dots. The photoluminescence emitted from thequantum dots and the light from the LED chip not absorbed by the quantumdots are mixed to produce white light.

In these patent application documents, use of quantum dots made of aGroup 12-Group 16 material, such as CdSe or CdTe, or a Group 14-Group 16material, such as PbS or PbSe, is proposed.

One of the advantages of using quantum dots in light-emitting devices isthat light with a wavelength corresponding to a bandgap of the quantumdots can have a peak with a relatively narrow full width at halfmaximum. Of the quantum dots proposed as the wavelength conversionmaterial, quantum dots made of a binary semiconductor, typified by Group12-14 semiconductors, such as CdSe, are confirmed to emit the light withthe wavelength corresponding to the bandgap of the quantum dots, i.e.,band-edge emission. Meanwhile, ternary quantum dots, in particular,Group 11-13-16 quantum dots have not been confirmed to exhibit theband-edge emission.

The light emission from the Group 11-13-16 quantum dots is caused by thedefect levels at the surface or inside of the particles, or by thedonor-acceptor-pair recombination, so that such light has a broadphotoluminescence peak with a wide full width at half maximum and a longphotoluminescence lifetime. Such light emission is not suitable forlight-emitting devices, particularly, used in a liquid crystal displaydevice. This is because a light-emitting device used in a liquid crystaldisplay device is required to emit light with a narrow full width athalf maximum that has a peak wavelength corresponding to each of threeprimary colors (i.e., RGB) in order to ensure high colorreproducibility. For this reason, practical applications of the ternaryquantum dots have not been prompted despite their less toxiccompositions.

SUMMARY

Therefore, one object of certain embodiments of the present disclosureis to provide semiconductor nanoparticles that are configured to produceband-edge emission from ternary quantum dots of which composition can beless toxic, and a method of producing the semiconductor nanoparticles.

According to certain embodiments of the disclosure, a method ofproducing semiconductor nanoparticles, the method includes:

(a) providing a salt of Ag, a salt of In, a source compound of S, and anorganic solvent; and

(b) placing the salt of Ag, the salt of In, and the source compound of Sinto the organic solvent so that the ratio of the number of atoms of Agto the total number of atoms of Ag and In is 0.33 or more and 0.42 orless to obtain semiconductor nanoparticles.

According to other certain embodiments of the disclosure, a method ofproducing core-shell semiconductor nanoparticles includes:

providing a dispersion in which semiconductor nanoparticles that areproduced by the method of producing semiconductor nanoparticlesaccording to the above embodiments are dispersed into a solvent,

adding, to the dispersion, a compound containing a Group 13 element andan elemental substance of a Group 16 element or a compound containing aGroup 16 element, to form a semiconductor layer consisting essentiallyof the Group 13 element and the Group 16 element on a surface of each ofthe semiconductor nanoparticles.

According to further other certain embodiments of the disclosure,semiconductor nanoparticles contain Ag, In, and S, and have an averageparticle size of 50 nm or less.

The ratio of the number of atoms of Ag to the total number of atoms ofAg and In is 0.320 or more and 0.385 or less.

The ratio of the number of atoms of S to the total number of atoms of Agand In is 1.20 or more and 1.45 or less.

The semiconductor nanoparticles are adapted to emit photoluminescencehaving a photoluminescence lifetime of 200 ns or less upon beingirradiated with light having a wavelength in a range of 350 to 500 nm.

According to further other certain embodiments of the disclosure, acore-shell semiconductor nanoparticle includes a core and a shellcovering a surface of the core and being in heterojunction with thecore.

The core is made of a semiconductor that contains Ag, In, and S, whereinthe ratio of the number of atoms of Ag to the total number of atoms ofAg and In is 0.320 or more to 0.385 or less, and the ratio of the numberof atoms of S to the total number of atoms of Ag and In is 1.20 or moreand 1.45 or less.

The shell is a semiconductor that consists essentially of a Group 13element and a Group 16 element.

In a photoluminescence spectrum obtained when the core-shellsemiconductor nanoparticle is irradiated with light having a wavelengthin a range of 350 to 500 nm, a photoluminescence peak having a peakwavelength in a range of 550 nm to 650 nm and a full width at halfmaximum of 80 nm or less is observed.

According to further other certain embodiments of the disclosure, alight-emitting device including a light conversion member and asemiconductor light-emitting element, wherein the light conversionmember contains the above semiconductor nanoparticles according tocertain embodiments of the disclosure.

The semiconductor nanoparticles according to certain embodiments of thedisclosure is made of a semiconductor that contains Ag, In, and S, whichcorrespond to a Group 11 element, a Group 13 element, and a Group 16element, respectively, wherein the ratio of the number of atoms of Ag tothe total number of atoms of Ag and In and the ratio of the number ofatoms of S to the total number of atoms of Ag and In are respectively ina specific range, and photoluminescence emission with a shortphotoluminescence lifetime, i.e., the band-edge emission, is obtained.The semiconductor nanoparticles can have composition that containsneither Cd nor Pb, which are highly toxic. Thus, the semiconductornanoparticles can be applied to products that are prohibited from usingCd or the like. Therefore, such semiconductor nanoparticles are suitablefor use as a wavelength conversion material for light-emitting devicesused in the liquid crystal display device or the like or as abiomolecule marker. Furthermore, with the method of producingsemiconductor nanoparticles as described above, the ternarysemiconductor nanoparticles that exhibit band-edge emission can berelatively easily produced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an X-ray diffraction (XRD) pattern of semiconductornanoparticles in Example 2.

FIG. 2 shows absorption spectra of the semiconductor nanoparticles inExamples 1 to 3.

FIG. 3 shows photoluminescence spectra of the semiconductornanoparticles in Examples 1 to 3.

FIG. 4 shows absorption spectra of the semiconductor nanoparticles inExample 4 and Comparative Examples 1 to 2.

FIG. 5 shows photoluminescence spectra of the semiconductornanoparticles in Example 4 and Comparative Examples 1 and 2.

FIG. 6 shows a photoluminescence/absorption spectrum of the cores(primary semiconductor nanoparticles) produced in Example 5.

FIG. 7 shows a photoluminescence/absorption spectrum of the core-shellsemiconductor nanoparticles produced in Example 5.

FIG. 8 shows a high-angle annular dark field (HAADF) image of thecore-shell semiconductor nanoparticles produced in Example 5.

FIG. 9 shows an HAADF image of the cores (primary semiconductornanoparticles) produced in Example 5.

DETAILED DESCRIPTION

Certain embodiments of the present disclosure will be described indetail below. Note that the present disclosure is not intended to belimited by the following embodiments. Description in one embodiment andits variant example can be applied to other embodiments and variantexamples unless otherwise specified.

First Embodiment: Semiconductor Nanoparticles

As the first embodiment, semiconductor nanoparticles including Ag, In,and S will be described.

Semiconductor nanoparticles in the present embodiment are semiconductornanoparticles that contain Ag, In, and S, and have an average particlesize of 50 nm or less. The crystal structure of the semiconductornanoparticles may be at least one selected from the group consisting ofa tetragonal system, a hexagonal system, and an orthorhombic system.

A semiconductor nanoparticle containing the above specific elements andhaving the crystal structure of the tetragonal system, hexagonal system,or orthorhombic system is generally represented by a composition formulaof AgInS₂, as indicated in various literature and the like. Of thesemiconductors represented by the composition formula of AgInS₂, asemiconductor having the hexagonal system is of the wurtzite type, and asemiconductor having the tetragonal system is of the chalcopyrite type.The crystal structure is identified, for example, by measuring an X-raydiffraction (XRD) pattern obtained by X-ray diffraction. Morespecifically, an XRD pattern obtained from the semiconductornanoparticles is compared with a known XRD pattern of semiconductornanoparticles represented by the composition formula of AgInS₂, or anXRD pattern determined by simulation from crystal structure parameters.If one of the known patterns or simulated patterns is identical to thepattern of the semiconductor nanoparticles, the semiconductornanoparticles can be regarded to have the same crystal structure as thatof the identical known or simulated pattern.

The aggregate of the nanoparticles may contain nanoparticles withdifferent crystal structures. In that case, the XRD pattern having peaksderived from a plurality of crystal structures is observed.

To obtain a semiconductor represented by a composition formula ofAgInS₂, generally the semiconductor is synthesized by selecting theproportion (charging ratio) of Ag source, In source, and S sourcecompounds so that it corresponds to the stoichiometric composition. Theinventors have studied the possibility of obtaining the band-edgeemission when, in a semiconductor consisting of Ag, In, and S, or asemiconductor that contains these elements, these elements have acomposition out of the stoichiometric compositional ratio. As a result,the inventors have found that when semiconductor nanoparticles areproduced by selecting the charging ratio of each element source so thatthe ratio of the number of atoms of Ag to the total number of atoms ofAg and In is 0.33 or more and 0.42 or less as mentioned later,semiconductor nanoparticles that enable the band-edge emission can beobtained despite of the non-stoichiometric composition.

In the semiconductor nanoparticles in the present embodiment, the ratioof the number of atoms of Ag to the total number of atoms of Ag and In(Ag/(Ag+In)) is 0.320 or more and 0.385 or less, and the ratio of thenumber of atoms of S to the total number of atoms of Ag and In(S/(Ag+In)) is 1.20 or more and 1.45 or less. The fact that Ag/(Ag+In)is 0.320 or more and 0.385 or less means that In is included in aproportion higher than the stoichiometric compositional ratio. The factthat S/(Ag+In) is 1.20 or more and 1.45 or less means that S is includedin a proportion higher than the stoichiometric compositional ratio. IfAg, In, and S are included as per the stoichiometric composition,Ag/(Ag+In) becomes 0.5, and S/(Ag+In) becomes 1.

Although the reason why that the semiconductor nanoparticles in thepresent embodiment exhibit a relatively intense band-edge emission hasnot been clear, for example, it is also assumed that In and S formIn₂S₃, which may give certain effects. This assumption does not limitthe invention described in the claims.

In the present embodiment, Ag/(Ag+In) is 0.320 or more and 0.385 orless, and particularly 0.350 or more and 0.382 or less, and S/(Ag+In) is1.20 or more and 1.45 or less, and particularly 1.25 or more and 1.40 orless. When Ag/(Ag+In) and S/(Ag+In) are respectively in these ranges, aband-edge emission is easily obtained, or a band-edge emission withhigher light intensity is easily obtained.

The chemical composition of the semiconductor nanoparticles can beidentified by, for example, energy dispersive X-ray spectrometry orX-ray fluorescence analysis (XRF). Ag/(Ag+In) and S/(Ag+In) arecalculated based on the chemical composition measured by either of thesemethods.

The nanoparticles in the first embodiment may be substantially made ofonly Ag, In, and S. Note that the term “substantially” as used herein isused in view of possible presence of one or greater elements other thanthe elements Ag, In, and S that may be unintentionally mixed in asimpurities or the like. Alternatively, the nanoparticles in the firstembodiment may contain other elements, as long as Ag/(Ag+In) andS/(Ag+In) are in the above ranges.

The semiconductor nanoparticles in the present embodiment have anaverage particle size of 50 nm or less. The average particle size may bein a range of 1 nm to 20 nm, and particularly in a range of 1 nm to 10nm. When the average particle size is over 50 nm, the quantum sizeeffect becomes not easily exhibited, and the band-edge emission becomesnot easily obtained.

The average particle size of the nanoparticles may be determined, forexample, from a TEM image taken with a transmission electron microscope(TEM). More specifically, the particle size of a nanoparticle refers to,in the TEM image, the longest one of line segments that connects any twopoints at the outer periphery of a single particle and passes throughthe center of the particle.

In the case where the particle has a rod shape, the particle sizethereof refers to the length of the short axis. In the presentspecification, the expression “rod-shaped” refers to a shape that isobserved to have a quadrangular shape such as a rectangular shape (witha cross section of a circular shape, an elliptical shape, or a polygonalshape), an elliptical shape, or a polygonal shape (for example, apencil-like shape), and have a ratio of the long axis to the short axisof more than 1.2. The “long axis” of a rod-shaped particle having anelliptical shape refers to the length of the longest one of linesegments each connecting any two points on the periphery of theparticle. Also, the “long axis” of a rod-shaped particle having aquadrangular or polygonal shape refers to the length of the longest oneof line segments that are in parallel with the longest side of the sidesdefining the periphery of the particle and connect two points on theperiphery of the particle. Meanwhile, the “short axis” indicates thelength of the longest one of line segments that are perpendicular to theline segment defining the long axis and connect two points on theperiphery of the particle.

The average particle size is an arithmetic average of the particle sizesdetermined by measuring the particle sizes of all the measurablenanoparticles observed in a TEM image at a magnification of 50,000× to150,000×. Here, the “measurable particle” refers to a particle which canbe observed as a whole in the TEM image. Thus, a particle that ispartially “cut” or not included in an image range is not regarded as a“measurable particle”.

In the case where the total number of nanoparticles included in one TEMimage is 100 or more, the average particle size is determined using oneTEM image. When the number of nanoparticles included in one TEM image issmall, the site to be captured is changed to obtain another TEMimage(s), and the particle sizes of 100 or more particles included intwo or more TEM images are measured.

The semiconductor nanoparticles in the present embodiment enable theband-edge emission due to Ag/(Ag+In) and S/(Ag+In) being in the aboveranges. More specifically, the semiconductor nanoparticles in thepresent embodiment can emit photoluminescence having a longer wavelengththan that of the irradiated light and having a photoluminescencelifetime of 200 ns or less, upon being irradiated with light having awavelength in a range of 350 nm to 500 nm. The photoluminescenceemission with a photoluminescence lifetime of 200 ns or less ispreferably observed as one having a full width at half maximum of 150 nmor less in the photoluminescence spectrum exhibited by the semiconductornanoparticles.

The value of “photoluminescence lifetime” is determined in the proceduredescribed below. First, semiconductor nanoparticles are irradiated withan excitation light to emit photoluminescence. Regarding light withwavelengths around the peak of the emission spectrum, for example, in arange of (peak wavelength ±50 nm), a change in the decay of the light(afterglow) is measured over time. The measurement of the change overtime starts from a timing when the irradiation with the excitation lightis stopped. In general, a decay curve is the sum of a plurality of decaycurves derived from relaxation processes of such as photoluminescenceemission or heat. Thus, in the present embodiment, on the assumptionthat the obtained decay curve contains three components (i.e., threedecay curves), parameter fitting is performed such that thethree-component decay curve is represented by the following formulawhere I(t) represents intensity of photoluminescence. The parameterfitting is performed using a dedicated software.I(t)=A ₁ exp(−t/τ ₁)+A ₂ exp(−t/τ ₂)+A ₃ exp(−t/τ ₃)

In the above formula, each of τ₁, τ₂, and τ₃ of the componentsrepresents the time required for attenuation of the light intensity to1/e (36.8%) of the corresponding initial level, which corresponds to thephotoluminescence lifetime of each component. The times τ₁, τ₂, and τ₃are named in the increasing order of the photoluminescence lifetime. A₁,A₂, and A₃ represent contribution rates of the respective components. Inthe present embodiment, when the component having the maximum integralof the curve represented by A_(x)exp(−t/τ_(x)) is assumed as the maincomponent, τ of the main component is regarded as a photoluminescencelifetime of light whose photoluminescence lifetime is measured (lighthaving a wavelength around the peak). Photoluminescence with aphotoluminescence lifetime of 200 ns or less is assumed to be theband-edge emission. When identifying the main component, the valuesA_(x)×τ_(x) of the components, which are obtained by integratingA_(x)exp(−t/τ_(x)) from 0 to infinity with respect to t, are compared toone another, and then the component with the largest A_(x)×τ_(x) isdetermined as the main component.

In this embodiment, the decay curves obtained by performing theparameter fitting assuming that the decay curve contains three, four, orfive components, respectively, do not greatly differ from each other interms of the deviation from an actual decay curve. For this reason, inthe present embodiment, the number of components included in the decaycurve of photoluminescence is assumed to be three when τ of the maincomponent is determined, allowing for avoiding the complexity of theparameter fitting.

The photoluminescence spectrum of the semiconductor nanoparticles in thepresent embodiment is obtained when the semiconductor nanoparticles areirradiated with light having one certain wavelength selected from therange of 350 nm to 1100 nm. For example, when nanoparticles having thecrystal structure of the tetragonal system in which Ag/(Ag+In) is 0.361and S/(Ag+In) is 1.30 (corresponding to Example 2 mentioned later) areirradiated with light having a wavelength of 365 nm, a photoluminescencespectrum can be obtained in which a photoluminescence peak derived fromthe band-edge emission is observed at around 590 nm, as shown by thedotted line in FIG. 3.

The semiconductor nanoparticles in the present embodiment may exhibit,together with the band-edge emission, other types of photoluminescence,for example, defect luminescence. The defect luminescence generally hasa long photoluminescence lifetime and shows a broad spectrum whose peakis positioned on the longer-wavelength side with respect to theband-edge emission. When both band-edge emission and defect luminescenceare obtained, the intensity of the band-edge emission is preferablyhigher than that of the defect luminescence.

The position of the peak of the band-edge emission from thesemiconductor nanoparticles in the present embodiment can be changed byadjusting the shape and/or average particle size, particularly averageparticle size, of the semiconductor nanoparticles. For example, bydecreasing the average particle size of the semiconductor nanoparticles,the bandgap energy is further increased by the quantum size effect, andthe peak wavelength of the band-edge emission can be shifted toward theshort-wavelength side.

The absorption spectrum of the semiconductor nanoparticles in thepresent embodiment can be obtained by irradiating the semiconductornanoparticles with light having a wavelength selected from thepredetermined ranges. For example, when nanoparticles having the crystalstructure of the tetragonal system in which Ag/(Ag+In) is 0.361 andS/(Ag+In) is 1.30 (corresponding to Example 2 mentioned later) areirradiated with light having a wavelength of 190 nm to 1100 nm, anabsorption spectrum as shown by the solid line in FIG. 2 can beobtained. FIG. 2 shows an absorption spectrum at a wavelength in a rangeof about 350 nm to 750 nm, among the absorption spectra at a wavelengthof 190 nm to 1100 nm.

The semiconductor nanoparticles of the present embodiment preferablyhave absorption spectrum in which an exciton peak is present. Theexciton peak is a peak obtained by generation of an exciton. Thepresence of this peak in the absorption spectrum indicates that thesemiconductor nanoparticles have the small distribution of particlesizes and has less crystal defects, and thus they are suitable for theband-edge emission. The sharper the exciton peak is, the greater amountof particles of uniform particle size and less crystal defects theaggregate of semiconductor nanoparticles contains. This is assumed tonarrow the full width at half maximum of the photoluminescence from thenanoparticles, thereby improving the luminous efficiency. In theabsorption spectrum of the semiconductor nanoparticles in thisembodiment, the exciton peak is observed, for example, in a regionbetween 350 nm and 650 nm.

The surface of the semiconductor nanoparticles in the present embodimentmay be modified by any appropriate compound. A compound that modifiesthe surface of the nanoparticles is also called “surface modifier”. Asurface modifier is used for, for example, stabilizing the nanoparticle,preventing the agglomeration and growth of the nanoparticles, and/orimproving the dispersibility of the nanoparticles in the solvent.

In the present embodiment, examples of a surface modifier may include anitrogen-containing compound comprising a hydrocarbon group with acarbon number of 4 to 20, a sulfur-containing compound comprising ahydrocarbon group with a carbon number of 4 to 20, and anoxygen-containing compound comprising a hydrocarbon group with a carbonnumber of 4 to 20. Examples of the hydrocarbon group with the carbonnumber of 4 to 20 can include saturated aliphatic hydrocarbon groups,such as an n-butyl group, an isobutyl group, an n-pentyl group, ann-hexyl group, an octyl group, a decyl group, a dodecyl group, ahexadecyl group, and an octadecyl group; unsaturated aliphatichydrocarbon groups, such as an oleyl group; alicyclic hydrocarbongroups, such as a cyclopentyl group and a cyclohexyl group; and aromatichydrocarbon groups, such as a phenyl group, a benzyl group, a naphthylgroup, and a naphthylmethyl group. Among them, the saturated aliphatichydrocarbon groups and the unsaturated aliphatic hydrocarbon groups arepreferable. Examples of nitrogen-containing compounds include amines andamides. Examples of sulfur-containing compounds include thiols. Examplesof oxygen-containing compounds include fatty acids and the like.

The surface modifier is preferably a nitrogen-containing compoundcomprising the hydrocarbon group with the carbon number of 4 to 20.Examples of such a nitrogen-containing compound include alkyl amines,such as n-butyl amine, isobutyl amine, n-pentyl amine, n-hexyl amine,octyl amine, decyl amine, dodecyl amine, hexadecyl amine, and octadecylamine, and alkenyl amines, such as oleyl amine.

For the surface modifier, a sulfur-containing compound comprising thehydrocarbon group with the carbon number of 4 to 20 is preferably used.Examples of such a sulfur-containing compound include n-butanethiol,isobutanethiol, n-pentanethiol, n-hexanethiol, octanethiol, decanethiol,dodecanethiol, hexadecanethiol, and octadecanethiol.

For the surface modifier, a combination of two or more different surfacemodifiers may be used. For example, one compound (e.g., oleylamine)selected from the nitrogen-containing compounds exemplified above, andone compound (e.g., dodecanethiol) selected from the sulfur-containingcompounds exemplified above may be used in combination.

Second Embodiment: Method of Producing Semiconductor Nanoparticles

Next, as the second embodiment, a method of producing the semiconductornanoparticles in the first embodiment will be described. The productionmethod in the present embodiment is a method of producing semiconductornanoparticles, the method including:

(a) providing the salt of Ag, the salt of In, the source compound of S,and the organic solvent; and

(b) producing semiconductor nanoparticles by placing the salt of Ag, thesalt of In, and the source compound of S into the organic solvent sothat the ratio of the number of atoms of Ag to the total number of atomsof Ag and In is 0.33 or more and 0.42 or less.

This production method is characterized by placing the salt of Ag, thesalt of In, and the source compound of S into the organic solvent sothat the ratio of the number of atoms of Ag to the total number of atomsof Ag and In (Ag/Ag+In) is 0.33 or more and 0.42 or less. By placing thesource of each element so that Ag/(Ag+In) is in the above-mentionedrange, Ag—In—S semiconductor nanoparticles in which Ag/(Ag+In) is in therange described in the first embodiment can be obtained. In other words,the production method in the present embodiment is characterized byproducing semiconductor nanoparticles with the charging ratio of thesource of each element at a specific ratio that does not correspond tothe stoichiometric compositional ratio.

First Method of Producing the Semiconductor Nanoparticles

For example, the semiconductor nanoparticles may be produced by a methodin which a salt of Ag, a salt of In, and, as a source compound of S, acompound that may form a complex wherein S is a coordination element aremixed to give a complex, and then the complex is heat-treated. Any kindsof a salt of Ag and a salt of In may be employed, and either an organicacid salt or an inorganic acid salt may be employed. More specifically,the salt may be any one of a nitrate salt, an acetate salt, ahydrosulfate salt, a hydrochloride salt, and a sulfonate salt, and ispreferably an organic acid salt, such as an acetate salt. This isbecause the organic acid salt has high solubility in an organic solvent,which allows the reaction to proceed uniformly.

Examples of the source compound of S used in this production methodinclude β-dithiones, such as 2,4-pentanedithione; dithiols, such as1,2-bis(trifluoromethyl)ethylene-1,2-dithiol; diethyldithiocarbamate;and thiourea.

The complex is obtained by mixing the salt of Ag, the salt of In, andthe source compound of S. The complex may be formed by a method in whichthe salt of Ag, the salt of In, and the source compound of S are placedinto water or an organic solvent (in particular, an organic solvent withhigh polarity, such as ethanol, methanol, or acetone) and then mixed.

The organic solvent may be a surface modifier itself, or may contain asurface modifier. The surface modifier is as described in relation tothe first embodiment. Particularly, it is preferable to use an organicsolvent that contains at least one solvent selected from thiolscomprising a hydrocarbon group with the carbon number of 4 to 20 and atleast one solvent selected from amines comprising a hydrocarbon groupwith the carbon number of 4 to 20, or that consists of a combinationthereof.

In the production method accompanied by the formation of a complex, thesolvent is selected so that the complex is formed. For example, whenwater or the organic solvent cannot dissolve the salt of Ag, the salt ofIn, and the source compound of S, the complex is not formed, or thecomplex becomes not easily formed. Nevertheless, even when the organicsolvent cannot dissolve the salt of Ag, the salt of In, and the sourcecompound of S, some combinations of the source compound of each elementmay lead to the production of semiconductor nanoparticles without afunction of the source compound of S as a ligand, as described in“Method of Producing the Second Semiconductor Nanoparticles” below.

Here, the salt of Ag, the salt of In, and the source compound of S areused in the amount so that the ratio of the number of atoms of Ag to thetotal number of atoms of Ag and In (Ag/(Ag+In)) is 0.33 or more and 0.42or less. The source compound of S is preferably used in the amount sothat the ratio of the number of atoms of S to the total number of atomsof Ag and In (S/(Ag+In)) is 0.95 or more and 1.20 or less. By using thesource compound of each element to meet these conditions, semiconductornanoparticles that easily exhibit the band-edge emission or that havehigh intensity of the band-edge emission can be produced.

Ag/(Ag+In) is more preferably 0.38 or more and 0.42 or less, andparticularly 0.40. S/(Ag+In) is more preferably 0.95 or more and 1.10 orless, and particularly 1.00.

Next, the obtained complex is subjected to heat treatment to form thesemiconductor nanoparticles. The heat treatment of the complex may beperformed by placing the salts and the source compound of S into theorganic solvent, which is a surface modifier or a solvent containing thesurface modifier, and then subjected to the heat treatment, which allowsfor continuously or simultaneously performing the complex formation, theheat treatment, and the surface modification.

The above heat treatment is preferably performed at a temperature of230° C. or more and 260° C. or less for 3 minutes or more. The heatingtime is more preferably 5 minutes or more, still more preferably 8minutes or more, and most preferably 10 minutes or more. More preferableheating temperature is 245° C. or more and 255° C. or less, andparticularly may be 250° C. The heating time is, for example, 60 minutesor less, and particularly 30 minutes or less. The heating time may be 10minutes.

Alternatively, the above heat treatment may be performed at atemperature of 30° C. or more and 190° C. or less for 1 minute or moreand 15 minutes or less, and then at a temperature of 230° C. or more and260° C. or less for 3 minutes or more. When the heat treatment isperformed in two steps, semiconductor nanoparticles having higherintensity of the band-edge emission are easily obtained. In this case,the heating temperature in the first step is more preferably 45° C. to155° C., still more preferably 145° C. to 155° C., and particularly maybe 150° C. The heating time in the first step is more preferably 8minutes or more and 13 minutes or less, still more preferably 9 minutesor more and 11 minutes or less, and particularly 10 minutes. The heatingtime in the second step is more preferably 5 minutes or more, still morepreferably 8 minutes or more, and most preferably 10 minutes or more.The heating temperature in the second step is more preferably 245° C. ormore and 255° C. or less, and particularly may be 250° C. The heatingtime in the second step is, for example, 60 minutes or less, andparticularly 30 minutes or less. The heating time may be 10 minutes.

The heat treatment in two steps can produce semiconductor nanoparticleshaving relatively high intensity of the band-edge emission with goodreproducibility.

Second Method of Producing the Semiconductor Nanoparticles

The semiconductor nanoparticles may be produced by a method in which thesalt of Ag, the salt of In, and the source compound of S into an organicsolvent at once and then the organic solvent is heated. With thismethod, nanoparticles can be synthesized in one pot with goodreproducibility by simple operations.

Alternatively, the semiconductor nanoparticles may be produced by amethod that includes forming a complex by a reaction between the organicsolvent and the salt of Ag, forming another complex by a reactionbetween the organic solvent and the salt of In, reacting these complexeswith the source compound of S to produce a reaction product, and growingcrystals of the obtained reaction product. In this case, heating isperformed in the step of reacting the source compound of S.

The salt of Ag and the salt of In have been described above in relationto the production method including the formation of complex (the firstmethod of producing the semiconductor nanoparticles).

Examples of the organic solvent include amines comprising a hydrocarbongroup with the carbon number of 4 to 20, particularly alkylamines oralkenylamines with the carbon number of 4 to 20; thiols comprising ahydrocarbon group with the carbon number of 4 to 20, particularlyalkylthiols or alkenylthiols with the carbon number of 4 to 20;phosphines comprising a hydrocarbon group with the carbon number of 4 to20, particularly alkylphosphines or alkenylphosphines with the carbonnumber of 4 to 20. These organic solvents finally serve to modify thesurfaces of the obtained semiconductor nanoparticles. Two or more ofthese organic solvents may be used in combination. In particular, amixed solvent that contains at least one solvent selected from thiolscomprising a hydrocarbon group with the carbon number of 4 to 20 and atleast one solvent selected from amines comprising a hydrocarbon groupwith the carbon number of 4 to 20 may be used. Such organic solvents maybe used mixed with other organic solvents.

In the second production method, examples of a source compound of Sinclude sulfur, thiourea, thioacetamide, and alkylthiol.

Also when this production method is employed, the salt of Ag, the saltof In, and the source compound of S are used in the amount so that theratio of the number of atoms of Ag to the total number of atoms of Agand In (Ag/(Ag+In)) is 0.33 or more and 0.42 or less. The sourcecompound of S is preferably used in the amount so that the ratio of thenumber of atoms of S to the total number of atoms of Ag and In(S/(Ag+In)) is 0.95 or more and 1.20 or less. By using the sourcecompound of each element to meet these conditions, semiconductornanoparticles that easily exhibit the band-edge emission can beproduced.

Ag/(Ag+In) is more preferably 0.38 or more and 0.42 or less, andparticularly 0.40. S/(Ag+In) is more preferably 0.95 or more and 1.10 orless, and particularly 1.00.

The heat treatment is preferably performed at a temperature of 230° C.or more and 260° C. or less for 3 minutes or more. The heating time ismore preferably 5 minutes or more, still more preferably 8 minutes ormore, and most preferably 10 minutes or more. More preferable heatingtemperature is 245° C. or more and 255° C. or less, and particularly maybe 250° C. The heating time is, for example, 60 minutes or less, andparticularly 30 minutes or less. The heating time may be 10 minutes.

Alternatively, the heat treatment may be performed at a temperature of30° C. or more and 190° C. or less for 1 minute or more and 15 minutesor less, and then at a temperature of 230° C. or more and 260° C. orless for 3 minutes or more. When the heat treatment is performed in twosteps, semiconductor nanoparticles having higher intensity of theband-edge emission are easily obtained. In this case, the heatingtemperature in the first step is more preferably 45° C. to 155° C.,still more preferably 145° C. to 155° C., and particularly may be 150°C. The heating time in the first step is more preferably 8 minutes ormore and 13 minutes or less, still more preferably 9 minutes or more and11 minutes or less, and particularly 10 minutes. The heating time in thesecond step is more preferably 5 minutes or more, still more preferably8 minutes or more, and most preferably 10 minutes or more. The heatingtemperature in the second step is more preferably 245° C. or more and255° C. or less, and particularly may be 250° C. The heating time in thesecond step is, for example, 60 minutes or less, and particularly 30minutes or less. The heating time may be 10 minutes.

The heat treatment in two steps can produce semiconductor nanoparticleshaving relatively high intensity of the band-edge emission with goodreproducibility.

Third Method of Producing the Semiconductor Nanoparticles

The semiconductor nanoparticles may be produced by using a so-calledhot-injection method. The hot-injection method is a method of producingsemiconductor nanoparticles in which a solution (also called as aprecursor solution) in which source compounds of respective elements(e.g., a salt of Ag, a salt of In, and a source compound of S) aredissolved or dispersed is charged into a solvent heated at a temperaturein a range of 100° C. to 300° C. for a short time (e.g. millisecondsorder), thereby forming a number of crystal nuclei in an initialreaction stage.

Alternatively, the hot-injection method may involve: dissolving ordispersing source compounds of some of the elements in an organicsolvent, followed by heating it; and subsequently charging a precursorsolution containing the remaining elements into the organic solvent. Inthe case where the solvent is the surface modifier, or a solventcontaining the surface modifier, the modification of the surfaces of theparticles can be performed simultaneously. The surface modifier is asdescribed in relation to the first embodiment.

Also in the hot-injection method, the source compounds of respectiveelements that are finally charged into a solvent are used in the amountso that the ratio of the number of atoms of Ag to the total number ofatoms of Ag and In (Ag/(Ag+In)) is 0.33 or more and 0.42 or less. Thesource compound of S is preferably used in the amount so that the ratioof the number of atoms of S to the total number of atoms of Ag and In(S/(Ag+In)) is 0.95 or more and 1.20 or less. By using the sourcecompound of each element to meet these conditions, semiconductornanoparticles that easily exhibit the band-edge emission or that havehigh intensity of the band-edge emission can be produced.

Ag/(Ag+In) is more preferably is 0.38 or more and 0.42 or less, andparticularly 0.40. S/(Ag+In) is more preferably 0.95 or more and 1.10 orless, and particularly 1.00.

The method of producing the semiconductor nanoparticles is not limitedto the above methods. Any appropriate method may be used, as long as theamount of the source compound of each element used is selected so thatAg/(Ag+In) is 0.33 or more and 0.42 or less.

In the case of employing any of these methods, the semiconductornanoparticles are produced under an inert atmosphere, particularly,under an argon atmosphere or a nitrogen atmosphere. This is for reducingor preventing the subgeneration of oxides and the oxidation of thesurface of the semiconductor nanoparticle.

In any of these production methods, when the heat treatment isperformed, the temperature increasing rate until the temperature reachesthe predetermined heating temperature (for example, temperature that ishold for a certain time) may be, for example, 1° C./min to 50° C./min.After heat treatment, cooling may be performed, for example, at atemperature decreasing rate of 1° C./min to 100° C./min so that thetemperature is decreased to the predetermined temperature.Alternatively, cooling after heat treatment may be performed by allowingto cool by turning off a heating source.

In any of these production methods, after the end of the production ofthe semiconductor nanoparticles, the obtained semiconductornanoparticles may be separated from the treated organic solvent, and maybe refined as needed. The separation is performed, for example, bycentrifuging an organic solvent containing the nanoparticles after theend of the production, and taking out a supernatant solution containingthe nanoparticles. The refinement includes, for example, adding anorganic solvent to the supernatant solution, centrifuging thesupernatant solution, and taking out the semiconductor nanoparticles asprecipitates. The precipitates themselves may be taken out, or theprecipitates may be taken out by removing the supernatant solution. Theprecipitates taken out may be dried, for example, by vacuum deaerationor natural drying, or alternatively, by a combination of the vacuumdeaeration and natural drying. The natural drying may be performed, forexample, by leaving the particles in the atmosphere at normaltemperature and pressure. In this case, the particles are left for 20hours or more, for example, for approximately 30 hours.

Alternatively, the precipitates taken out may be dissolved in an organicsolvent. The refinement (including the addition of alcohol and thecentrifugation) may be performed a plurality of times as needed.Examples of an alcohol for use in the refinement include a loweralcohol, such as methanol, ethanol, and n-propanol. In the case ofdissolving the precipitates in an organic solvent, examples of organicsolvents include chloroform, toluene, cyclohexane, hexane, pentane, andoctane.

The semiconductor nanoparticles obtained in this way tend to be obtainedin the form wherein the ratio of the number of atoms of Ag to the totalnumber of atoms of Ag and In is smaller than the ratio of the number ofatoms of Ag to the total number of atoms of Ag and In, in the salt of Agand the salt of In that are placed into an organic solvent. Therefore,according to the production method in the present embodiment, it ispossible to produce semiconductor nanoparticles including Ag, In, and S,wherein, for example, the ratio of the number of atoms of Ag to thetotal number of atoms of Ag and In is 0.320 or more and 0.385 or less,and the ratio of the number of atoms of S to the total number of atomsof Ag and In is 1.20 or more and 1.45 or less. The semiconductornanoparticles can emit photoluminescence having a photoluminescencelifetime of 200 ns or less upon being irradiated with light having awavelength in a range of 350 to 500 nm.

When the organic solvent is a surface modifier itself, or contains asurface modifier, semiconductor nanoparticles of which surface ismodified can be obtained.

Third Embodiment: Core-Shell Semiconductor Nanoparticles

As the third embodiment, a core-shell semiconductor nanoparticle thatincludes a core and a shell covering a surface of the core and being inheterojunction with the core will be described.

In the present embodiment, the core is the semiconductor nanoparticledescribed as the first embodiment. Thus, its detailed description willbe omitted.

The shell is formed of a semiconductor that consists essentially of aGroup 13 element and a Group 16 element. The semiconductor constitutingthe shell may be a semiconductor that has a bandgap energy larger than abandgap energy of the semiconductor constituting the core. Examples ofthe Group 13 element can include B, Al, Ga, In, and TI, and examples ofthe Group 16 element can include O, S, Se, Te, and Po.

When the semiconductor nanoparticle (core) in the first embodiment iscovered with the semiconductor (shell) that consists essentially of aGroup 13 element and a Group 16 element, the proportion of the band-edgeemission can be higher and the proportion of photoluminescence otherthan the band-edge emission (particularly defect luminescence) can belower, compared with when no shell is used. It is assumed that this isbecause covering the core with the shell results in no surface defectsite of the core. In the present embodiment, it is assumed that in thecase where the shell has a bandgap energy larger than that of the core,an energetic barrier is formed, resulting in a higher proportion of theband-edge emission.

Examples of a combination of a Group 13 element and a Group 16 elementof the shell may include a combination of Ga and S. The combination ofGa and S is preferably used since it has a larger bandgap energy. Thecombination of Ga and S may be gallium sulfide. In the presentembodiment, gallium sulfide forming the shell may not be stoichiometric(that is, Ga₂S₃), and thus may be represented by the formula of GaS_(x)(where x is any number that is not limited to an integer number, e.g.,in a range of 0.8 to 1.5) in the present specification.

The shell may be amorphous. Whether the amorphous shell is formed or notcan be confirmed by observing the core-shell semiconductor nanoparticlesof this embodiment with a high-angle annular dark field (HAADF)-scanningtransmission electron microscope (STEM). More specifically, in anHAADF-STEM image, a portion with a regular pattern (e.g., a stripedpattern or a dot pattern) is observed at the center, while a portionwith no regular pattern is observed at the periphery of the portion withthe regular pattern. With the HAADF-STEM, a regular structure, such ascrystal material, is observed as an image with a regular pattern, whilea non-regular structure, such as amorphous material, is not observed asan image with a regular pattern. For this reason, when the shell isamorphous, the shell can be observed as a part definitely different fromthe core, which is observed as an image with a regular pattern (as ithas the crystal structure, such as a tetragonal system, as mentionedabove).

When the core is made of Ag—In—S, and the shell is made of GaS, in theimage obtained by the HAADF-STEM, the shell tends to be observed as animage part darker than that of the core because Ga is an element with aweight smaller than each of Ag and In.

The surface of the shell may be modified by any appropriate compound. Inthe case where the surface of the shell is an exposed surface of thecore-shell semiconductor nanoparticle, modification of the exposedsurface allows for stabilizing the nanoparticle, and preventing theagglomeration and growth of the semiconductor nanoparticles, and/orimproving the dispersibility of the semiconductor nanoparticles in thesolvent. Applicable compounds as a surface modifier were as previouslydescribed in the first embodiment, and thus their description will beomitted here.

The core-shell semiconductor nanoparticles may have an average particlesize of 50 nm or less. The average particle size may be in a range of 1nm to 20 nm, and particularly in a range of 1 nm to 10 nm. The particlesize and the method of determining the average particle size are asdescribed in the first embodiment.

In the core-shell semiconductor nanoparticle, the core may have anaverage particle size of, for example, 10 nm or less, and particularly,8 nm or less. The average particle size of the core may be in a range of2 nm to 10 nm, particularly 2 nm to 8 nm, more particularly 2 nm to 6nm, or alternatively in a range of 4 nm to 10 nm or in a range of 5 nmto 8 nm. If the average particle size of the core is too large, thequantum size effect is not easily exhibited, so that band-edge emissionis not easily obtained.

The shell may have a thickness of 0.1 nm or more. The thickness of theshell may be in a range of 0.1 nm to 50 nm, particularly 0.1 nm to 20nm, and more particularly 0.2 nm to 10 nm. Alternatively, the thicknessof the shell may be in a range of 0.1 nm to 3 nm, and particularly 0.3nm to 3 nm. If the thickness of the shell is too small, the effectobtained by coating the core with the shell may not be sufficientlyexhibited, so that band-edge emission is not easily obtained.

The average particle size of the core and the thickness of the shell maybe determined by observing the core-shell semiconductor nanoparticle,for example, with the HAADF-STEM. In particular, in the case where theshell is amorphous, the shell is easily observed as a different partfrom the core, so that the thickness of the shell can be determinedeasily using the HAADF-STEM. In that case, the particle size of the corecan be determined by the method described above for that of thesemiconductor nanoparticle. In the case where the thickness of the shellis not uniform, the thickness of the shell in the particle refers to thesmallest thickness among the measured values.

Alternatively, the average particle size of the core may be measured inadvance before being coated with the shell. Then, an average particlesize of the core-shell semiconductor nanoparticles may be measured.Subsequently, a difference between the average particle size of thecore-shell semiconductor nanoparticles and the average particle size ofthe core measured in advance may be calculated to determine thethickness of the shell.

The core-shell semiconductor nanoparticles in the present embodiment arethe semiconductor nanoparticles of which core is as described in thefirst embodiment, and enable the band-edge emission due to Ag/(Ag+In)and S/(Ag+In) being in the above-mentioned particular ranges. Morespecifically, in a photoluminescence spectrum obtained when thecore-shell semiconductor nanoparticles are irradiated with light havinga wavelength in a range of 350 to 500 nm, a photoluminescence peakhaving a peak wavelength in a range of 550 nm to 650 nm and a full widthat half maximum of 80 nm or less, particularly 50 nm or less is observedat 25° C. The lower limit of the full width at half maximum may be, forexample, 20 nm, and particularly 10 nm.

The core-shell semiconductor nanoparticles in the present embodiment canemit the band-edge emission in a larger proportion since the abovespecific core is covered with the above specific shell. Morespecifically, the core-shell semiconductor nanoparticles in the presentembodiment can exhibit a photoluminescence spectrum wherein theintensity of the band-edge emission that is normalized by the intensityof the defect luminescence is, for example, 5 to 300, and particularly20 to 150. The core-shell semiconductor nanoparticles in the presentembodiment preferably do not emit light derived from the defectluminescence.

The photoluminescence lifetime of the band-edge emission exhibited bythe core-shell semiconductor nanoparticles in the present embodimenttends to be longer than that of semiconductor nanoparticles not coveredwith the shell (namely, the semiconductor nanoparticles in the firstembodiment), and may exceed 200 ns in some cases. This is because, dueto covering with the shell, the relaxation processes in whichnonradiative deactivation of excited electrons occurs are decreased, andthe lifetime of the excited electrons at band-edge levels is increased.The core-shell nanoparticles in the present embodiment are considered toexhibit the band-edge emission, as long as the full width at halfmaximum is 80 nm or less even when the photoluminescence lifetime of thephotoluminescence exceeds 200 ns.

Fourth Embodiment: Method of Producing Core-Shell SemiconductorNanoparticles

Next, as the fourth embodiment, a method of producing the core-shellsemiconductor nanoparticles in the third embodiment will be described.The method of producing the semiconductor nanoparticles each of which isto be the core is as described as the second embodiment.

In the covering of the semiconductor nanoparticles each of which is tobe the core (hereinafter the core before being coated with a shell isreferred to as “primary semiconductor nanoparticles” for convenience)with the shell, the primary semiconductor nanoparticles are dispersed inan appropriate solvent to prepare a dispersion. Subsequently, in thedispersion, a semiconductor layer is formed as the shell.Light-scattering does not occur in the liquid in which the primarysemiconductor nanoparticles are dispersed. Thus, the obtained dispersionis generally transparent (colored or colorless). Any appropriate solventmay be used for dispersing the primary semiconductor nanoparticlesthereinto. As in the production of the primary semiconductornanoparticles, an appropriate organic solvent (in particular, an organicsolvent with high polarity, such as ethanol) may be used. The organicsolvent may be the surface modifier or a solution containing the surfacemodifier. For example, the organic solvent may be at least one selectedfrom nitrogen-containing compounds comprising a hydrocarbon group withthe carbon number of 4 to 20; or at least one selected fromsulfur-containing compounds comprising a hydrocarbon group with thecarbon number of 4 to 20; or a combination of at least one selected fromthe nitrogen-containing compounds comprising a hydrocarbon group withthe carbon number of 4 to 20 and at least one selected from thesulfur-containing compounds comprising a hydrocarbon group with thecarbon number of 4 to 20. More specifically, examples of the organicsolvent include oleylamine, n-tetradecylamine, dodecanethiol, andcombinations thereof.

The dispersion of the primary semiconductor nanoparticles may beprepared such that the proportion of the nanoparticles in the dispersionis, for example, 0.02% by mass to 1% by mass, and particularly 0.1% bymass to 0.6% by mass. If the proportion of the particles in thedispersion is too low, products are difficult to be collected inaggregation and precipitation processes by a poor solvent. If theproportion of the particles in the dispersion is too high, rate ofOstwald ripening and fusion of particles of the materials forming thecore may be increased, which tends to widen the particle sizedistribution.

The formation of the semiconductor layer as the shell is performed byadding a compound containing a Group 13 element and either an elementalsubstance of a Group 16 element or a compound containing a Group 16element to the dispersion described above. The compound containing theGroup 13 element serves as a Group 13 element source, and is, forexample, an organic salt, an inorganic salt, an organometallic compoundof the Group 13, etc. More specifically, examples of the compoundcontaining the Group 13 element include nitrate salts, acetate salts,sulfate salts, hydrochloride salts, sulfonate salts, and acetylacetonatocomplexes, and preferably organic salts, such as acetate salts, ororganometallic compounds. This is because the organic salt andorganometallic compound have high solubility in the organic solvent,which can facilitate uniformly proceeding the reaction.

An elemental substance of the Group 16 element or a compound containingthe Group 16 element serves as a Group 16 element source. For example,in the case where sulfur (S) is a constituent element of the shell asthe Group 16 element, the elemental sulfur, such as high-purity sulfur,can be used, or sulfur-containing compounds can be used. Examples ofsuch sulfur-containing compounds include thiols, such as n-butanethiol,isobutanethiol, n-pentanethiol, n-hexanethiol, octanethiol, decanethiol,dodecanethiol, hexadecanethiol, and octadecanethiol; disulfides such asdibenzyldisulfide; thiourea; and thiocarbonyl compounds.

In the case where oxygen (O) is a constituent element of the shell asthe Group 16 element, alcohols, ethers, carboxylic acids, ketones, orN-oxide compounds may be used as a Group 16 element source. In the casewhere selenium (Se) is a constituent element of the shell as the Group16 element, an elemental selenium, or compounds, such as selenizedphosphine oxides, organic selenium compounds (dibenzyldiselenide,diphenyldiselenide) or selenium hydrides, may be used as a Group 16element source. In the case where tellurium (Te) is a constituentelement of the shell as the Group 16 element, an elemental tellurium,tellurium phosphine oxides, or tellurium hydrides, may be used as aGroup 16 element source.

Any appropriate method may be employed for adding the Group 13 elementsource and the Group 16 element source into the dispersion. For example,a mixture in which a Group 13 element source and a Group 16 elementsource may be dispersed or dissolved into an organic solvent may beprepared. Then, the mixture may be added to a dispersion little bylittle, for example, by dropping. In this case, the mixture may be addedat a rate of 0.1 mL/hr to 10 mL/hr, and particularly at a rate of 1mL/hr to 5 mL/hr. The mixture may be added to the heated dispersion.More specifically, for example, the temperature of the dispersion may beincreased to a peak temperature of 200° C. to 300° C. After reaching thepeak temperature, the mixture may be added little by little to thedispersion while holding the peak temperature, followed by decreasingthe temperature of the dispersion with the mixture added, so that ashell layer may be formed (namely, a slow injection method). The peaktemperature may be held if necessary even after the addition of themixture is finished.

If the peak temperature is too low, the surface modifier that modifiesthe primary semiconductor nanoparticles cannot be removed sufficiently,or a chemical reaction for forming the shell does not proceedsufficiently. For these reasons, the semiconductor layer (shell) is notformed sufficiently in some cases. On the other hand, if the peaktemperature is extremely high, the primary semiconductor nanoparticlessometimes have their quality altered, whereby even the formation of theshell cannot produce the band-edge emission in some cases. The time forholding the peak temperature may be set from one minute to 300 minutes,and particularly 10 minutes to 60 minutes in total after the start ofthe addition of the mixture. The time for holding at the peaktemperature (i.e., temperature holding time) is selected in view of therelationship with the peak temperature. When the peak temperature islower, the temperature holding time is increased. When the peaktemperature is higher, the temperature holding time is decreased. Inthese manners, good shell layer can be easily formed. The temperatureincreasing rate and the temperature decreasing rate are not particularlylimited. Decreasing of the temperature may be performed, for example, byholding the dispersion in which the mixture is added at the peaktemperature for a predetermined time and then allowing the dispersion tocool by turning off a heating source (e.g., electric heater).

Alternatively, whole amounts of the Group 13 element source and theGroup 16 element source may be added directly to the dispersion. Then,the dispersion to which the Group 13 element source and the Group 16element source are added may be heated, so that the semiconductor layercan be formed as the shell on the surfaces of the primary semiconductornanoparticles (namely, a heating-up method). More specifically, thetemperature of the dispersion into which the Group 13 element source andthe Group 16 element source are added may be gradually increased to apeak temperature of 200° C. to 300° C. After holding at the peaktemperature for one to 300 minutes, the temperature of the dispersionmay be gradually decreased. The temperature increasing rate may be set,for example, at 1° C./min to 50° C./min, and the temperature decreasingrate may be set, for example, at 1° C./min to 100° C./min.Alternatively, heating may be performed to a predetermined peaktemperature without controlling the temperature increasing rate.Alternatively, the temperature decreasing may not be performed at aconstant rate. The temperature decreasing may be performed by turningoff the heating source. The issues in the case of the excessively lowpeak temperature or excessively high peak temperature have beendescribed in description of the method of adding the mixture.

With the heating-up method, the core-shell semiconductor nanoparticlestend to be obtained, with band-edge emission of an intensity higher thanthat of the core-shell semiconductor nanoparticles in which the shell isformed by the slow-injection method.

The charging ratio of the Group 13 element source and the Group 16element source preferably corresponds to the stoichiometriccompositional ratio of the compound semiconductor consisting of theGroup 13 element and the Group 16 element in any method of adding theGroup 13 element source and the Group 16 element source. For example, inthe case of using a Ga source as the Group 13 element source and a Ssource as the Group 16 element source, the charging ratio is preferably1:1.5 (Ga:S) corresponding to a composition formula of Ga₂S₃. However,the charging ratio may not necessarily be the stoichiometriccompositional ratio. In the case where the raw material is charged withan excessive amount compared to the intended produced amount of theshell, the amount of the Group 16 element source may be smaller than thestoichiometric compositional ratio. For example, the charging ratio maybe 1:1 (the Group 13:the Group 16).

The charging amount is selected in view of an amount of the primarysemiconductor nanoparticles contained in a dispersion such that theshell with a desired thickness is formed on the primary semiconductornanoparticles contained in the dispersion. For example, the chargingamounts of the Group 13 element source and the Group 16 element sourcemay be selected such that the 0.01 mmol to 10 mmol, particularly 0.1mmol to 1 mmol of the compound semiconductor consisting of the Group 13element and the Group 16 element with stoichiometric composition issynthesized for the 10 nmol of the primary semiconductor nanoparticles,at amount of particles. The “amount of particles” refers to a molaramount assuming that the single particle is a giant molecule, and isequal to a value obtained by dividing the number of the nanoparticlescontained in the dispersion by Avogadro number (N_(A)=6.022×10²³).

In the case of using the heating-up method to form the shell, increasingthe temperature holding time (for example, 40 minutes or longer,particularly 50 minutes or longer, and the upper limit being, forexample, 60 minutes or shorter) allows for easily obtaining thecore-shell semiconductor nanoparticles with intense band-edge emissionand a higher proportion of the band-edge emission (a larger intensityratio of the band-edge emission/defect luminescence). However, when thetemperature holding time is too increased, the intensity itself of theband-edge emission tends to be decreased, and photoluminescence ishardly obtained in some cases. The more the temperature holding time isincreased, the more the peak of the band-edge emission emitted from theresultant semiconductor nanoparticles tends to be shifted toward theshorter-wavelength side, and the larger the full width at half maximumof the band-edge emission tends to be.

The shell is formed in this manner, so that the core-shell semiconductornanoparticles are formed. The obtained core-shell semiconductornanoparticles may be separated from the solvent. Further, the core-shellsemiconductor nanoparticles may be further refined and dried, if needed.The methods of separation, refinement and drying are as described in thesecond embodiment, and thus their detailed description will be omitted.

Light-Emitting Device

As another embodiment, a light-emitting device will be described inwhich the semiconductor nanoparticles which have been described above,namely, the semiconductor nanoparticles in the first embodiment, thesemiconductor nanoparticles produced by the method in the secondembodiment (including the first to the third methods of producing thesemiconductor nanoparticles), the core-shell semiconductor nanoparticlesin the third embodiment, or the core-shell semiconductor nanoparticlesproduced by the method in the fourth embodiment (hereinafter these arecollectively referred to as “the semiconductor nanoparticles of thepresent disclosure” or “the nanoparticles of the present disclosure”)are used. The light-emitting device according to the present embodimentis a light-emitting device including a light conversion member and asemiconductor light-emitting element, wherein the light conversionmember contains the semiconductor nanoparticles of the presentdisclosure. In such a light-emitting device, for example, thesemiconductor nanoparticles of the present disclosure absorb a portionof light emitted from the semiconductor light-emitting element, and thenemit light of a longer wavelength. Light from the semiconductornanoparticles of the present disclosure and the rest of light emittedfrom the semiconductor light-emitting element are mixed together, andthe mixed light can then be used as the light emission from thelight-emitting device.

More specifically, with the semiconductor light-emitting elementconfigured to emit blue-violet or blue light with a peak wavelength ofabout 400 nm to about 490 nm and the semiconductor nanoparticles of thepresent disclosure configured to absorb blue light and emit yellow lighttherefrom (for example, the semiconductor nanoparticles in the abovefirst embodiment having an average particle size of 4.0 to 5.0 nm), alight-emitting device configured to emit white light can be obtained.Alternatively, with two kinds of semiconductor nanoparticles of thepresent disclosure, of which one kind of semiconductor nanoparticlesconfigured to absorb blue light and emit green light and the other kindof semiconductor nanoparticles configured to absorb blue light and emitred light, a white light-emitting device can be obtained. Further,alternatively, with a semiconductor light-emitting element configured toemit ultraviolet light with a peak wavelength of 400 nm or less, andthree kinds of semiconductor nanoparticles of the present disclosurethat absorb the ultraviolet light and emit blue, green, and red lights,respectively, a white light-emitting device can be obtained. In thiscase, to avoid the leak of the ultraviolet light emitted from thelight-emitting element toward the outside, the entirety of light fromthe light-emitting element is desirably absorbed and converted by thesemiconductor nanoparticles of the present disclosure.

Moreover, alternatively, with a light-emitting element configured toemit a blue-green light with a peak wavelength of about 490 nm to 510 nmand the semiconductor nanoparticles of the present disclosure configuredto absorb the blue-green light and emit red light, a device configuredto emit white light can be obtained. Furthermore, alternatively, with asemiconductor light-emitting element configured to emit red light with awavelength of 700 nm to 780 nm and the semiconductor nanoparticles ofthe present disclosure configured to absorb the red light and emitnear-infrared light, a light-emitting device configured to emitnear-infrared light can be obtained.

The semiconductor nanoparticles of the present disclosure may be used incombination with other semiconductor quantum dots, or any phosphorsother than quantum dots (e.g., an organic phosphor or an inorganicphosphor). Other semiconductor quantum dots may be, for example, binarysemiconductor quantum dots mentioned in the Background section in thepresent specification. For the phosphors other than quantum dots,garnet-based phosphors, such as an aluminum garnet-based phosphor, canbe used. Examples of the garnet phosphors include anyttrium-aluminum-garnet-based phosphor activated by cerium, and alutetium-aluminum-garnet-based phosphor activated by cerium. Examples ofthe phosphors other than quantum dots include a nitrogen-containingcalcium aluminosilicate-based phosphor activated by europium and/orchromium; a silicate-based phosphor activated by europium; anitride-based phosphor such as a β-SiAlON-based phosphor, a CASN-basedphosphor, and a SCASN-based phosphor; a rare earth nitride-basedphosphor such as LnSi₃N₁₁-based phosphor and a LnSiAlON-based phosphor;an oxynitride-based phosphor such as a BaSi₂O₂N₂:Eu-based phosphor andBa₃Si₆O₁₂N₂:Eu-based phosphor; a sulfide-based phosphor such as aCaS-based phosphor, a SrGa₂S₄-based phosphor, a SrAl₂O₄-based phosphor,and a ZnS-based phosphor; a chlorosilicate-based phosphor; SrLiAl₃N₄:Euphosphor; SrMg₃SiN₄:Eu phosphor; and K₂SiF₆:Mn phosphor as a fluoridecomplex phosphor activated by manganese may be used.

The light conversion member which contains the semiconductornanoparticles of the present disclosure in the light-emitting device mayhave, for example, a sheet-like shape or a plate-like shaped member, ormay be a member with a three-dimensional shape. Examples of the memberwith the three-dimensional shape include a sealing member of asurface-mounted light-emitting diode in which a semiconductorlight-emitting element is arranged at a bottom surface of a recessdefined in a package. The sealing member is formed by charging resininto the recess to seal the light-emitting element.

Alternatively, other examples of the light conversion member include aresin member formed with a substantially uniform thickness to encloseupper surface and lateral surfaces of a semiconductor light-emittingelement arranged on a planar substrate. Further, alternatively, in thecase where a resin member which contains a reflective material is filledat the periphery of a semiconductor light-emitting element such that anupper end of the resin member is in the same plane with thesemiconductor light-emitting element, further other examples of thelight conversion member include a plate-shaped resin member with apredetermined thickness arranged on an upper side of the semiconductorlight-emitting element and the above-described resin member whichcontains the reflective material.

The light conversion member may be in contact with the semiconductorlight-emitting element, or may be disposed spaced from the semiconductorlight-emitting element. More specifically, the light conversion membermay be a pellet-shaped member, a sheet member, a plate-shaped member, ora stick-shaped member, which is disposed spaced from the semiconductorlight-emitting element. Alternatively, the light conversion member maybe a member which is disposed in contact with the semiconductorlight-emitting element for example, a seal member, a coating member(i.e., member covering the light-emitting element disposed spaced fromthe mold member), or a mold member (e.g., including a lens-shapedmember). In the case of using two or more kinds of semiconductornanoparticles of the present disclosure to exhibit photoluminescencewith different wavelengths in the light-emitting device, one lightconversion member may contain a mixture of the two or more kinds ofsemiconductor nanoparticles of the present disclosure, or alternatively,a combination of two or more light conversion members, each of whichcontains only one kind of quantum dots, may be used. In this case, twoor more kinds of light conversion members may be layered, or may bearranged on a plane in a dot-like or striped pattern.

Examples of the semiconductor light-emitting element include alight-emitting diode (LED) chip. The LED chip may include asemiconductor layer made of one or more kinds of compounds selected fromthe group consisting of GaN, GaAs, InGaN, AlInGaP, GaP, SiC, and ZnO.The semiconductor light-emitting element configured to emit blue-violetlight, blue light, or ultraviolet light preferably includes, as asemiconductor layer, a GaN based compound represented by a generalformula of In_(x)Al_(y)Ga_(1-x-y)N (0≤X, 0≤Y, X+Y<1).

The light-emitting device in this embodiment is preferably incorporatedas a light source in a liquid crystal display device. Since theband-edge emission produced by the semiconductor nanoparticles of thepresent disclosure has a short photoluminescence lifetime, thelight-emitting device using the semiconductor nanoparticles is suitablefor use as a light source for the liquid crystal display device thatrequires a relatively high response speed. The semiconductornanoparticles of the present disclosure can exhibit thephotoluminescence peak with a smaller full width at half maximum as theband-edge emission. Thus, the liquid crystal display device thatexhibits excellent color reproducibility can be obtained without using acolor filter with a dense color, if, in the light-emitting device:

the blue semiconductor light-emitting element is adapted to produce theblue light with a peak wavelength of 420 nm to 490 nm; and thesemiconductor nanoparticles of the present disclosure are adapted toproduce the green photoluminescence with a peak wavelength of 510 nm to550 nm, preferably 530 nm to 540 nm, and the red photoluminescence witha peak wavelength of 600 nm to 680 nm, preferably 630 nm to 650 nm,respectively; or

the semiconductor light-emitting element is adapted to produce theultraviolet light with a peak wavelength of 400 nm or less; and thesemiconductor nanoparticles of the present disclosure are adapted toproduce the blue photoluminescence with a peak wavelength of 430 nm to470 nm, preferably 440 nm to 460 nm, the green photoluminescence with apeak wavelength of 510 nm to 550 nm, preferably 530 nm to 540 nm, andthe red photoluminescence with a peak wavelength of 600 nm to 680 nm,preferably 630 nm to 650 nm, respectively. The light-emitting device ofthis embodiment is used, for example, as a direct illumination-typebacklight or an edge illumination-type backlight.

Alternatively, a sheet, a plate-shaped member, or a rod-shaped membermade of a resin, glass, or the like, which contains the semiconductornanoparticles of the present disclosure, may be incorporated as a lightconversion member independently from the light-emitting device, into aliquid crystal display device.

EXAMPLES Examples 1 to 3

First, 0.10 mmol of silver acetate (AgOAc), 0.15 mmol of indium acetate(In(OAc)₃), and 0.25 mmol of thiourea were placed into a mixture of 0.10cm³ of 1-dodecanethiol and 2.90 cm³ of oleylamine, and dispersed. In thesilver acetate and the indium acetate, Ag/(Ag+In) was 0.4 in all of theExamples. The dispersion was put into a test tube together with astirring bar, and nitrogen substitution was performed, followed by heattreatment under the conditions shown in Table 1, while stirring thecontent in the test tube under a nitrogen atmosphere

TABLE 1 Heat treatment in the Heat treatment in the first step secondstep 50° C. 150° C. 250° C. Example 1 10 min — 10 min Example 2 — 10 min10 min Example 3 — — 10 min

All of the heat treatments were performed at each of the predeterminedtemperature for 10 minutes. After heat treatment, the obtainedsuspension was allowed to cool, followed by centrifugal separation (witha radius of 146 mm at 4,000 rpm for 5 minutes), so that a supernatantsolution was taken out. Then, methanol was added to the supernatantsolution until nanoparticles were precipitated, followed by centrifugalseparation (with a radius of 146 mm at 4,000 rpm for 5 minutes), therebyprecipitating the nanoparticles. The precipitates were taken out anddissolved in chloroform, and the following measurement was performed.

An XRD pattern of the semiconductor nanoparticles obtained in Example 2was measured and compared with an XRD pattern of AgInS₂ having atetragonal system (of a chalcopyrite type) and an XRD pattern of AGInS₂having an orthorhombic system. The measured XRD pattern is shown inFIG. 1. The XRD pattern shows that the crystal structure of thesemiconductor nanoparticles in Example 2 is substantially the same asthe tetragonal AgInS₂. The XRD pattern was measured using a powder X-raydiffractometer manufactured by RIGAKU Corporation (trade name:SmartLab). Note that the same goes for the Examples in the belowdisclosures.

The shapes of the obtained semiconductor nanoparticles were observedusing a transmission electron microscope (TEM, manufactured by HITACHHIGH-TECHNOLOGIES Corporation, trade name: H-7650), and the averageparticle size was measured from TEM images at a magnification of 80,000×to 200,000×. Here, a commercially available Cu grid having an elasticcarbon supporting membrane (manufactured by OKENSHOJI Co., Ltd.) wasused as the TEM grid. The shape of the obtained particles was sphericalor polygonal.

An average particle size was determined by measuring the particle sizesof all measurable nanoparticles included in TEM images, i.e., allparticles except for nanoparticles whose images were cut at the edges ofthe images, and calculating an arithmetic average of the measuredparticle sizes. When the number of nanoparticles included in one TEMimage is less than 100, another TEM image was measured and the particlesizes of the particles included in this TEM image were measured, andthen the arithmetic average was calculated from 100 or more particles.

The average particle size of the semiconductor nanoparticles in eachExample was as follows:

Example 1: 4.1 nm

Example 2: 4.6 nm

Example 3: 5.1 nm

Next, the proportion of each of Ag, In and S atoms in the obtainedsemiconductor nanoparticles was determined using an energy dispersiveX-ray spectrometer (manufactured by HORIBA, Ltd., EMAX Energy), byassuming that the total number of atoms of Ag, In, and S included in thesemiconductor nanoparticles was 100. The result is shown in Table 2.

Measurement using an energy dispersive X-ray spectrometer wasspecifically performed in the following procedures. Note that the samegoes for the Examples in the below.

The produced nanoparticles-dispersed solution was dropped onto a carbontape fixed to a sample stage, and dried. Using signals derived from Ag,In, and S from the spectrum observed with an energy dispersive X-rayspectrometer, each component was quantitatively determined based on theintensity of the signals. Five-point measurement was performed whilechanging the measurement sites, and the average thereof was used as theresult.

TABLE 2 S Ag In Ag/(Ag + In) S/(Ag + In) Example 1 57.3 16.3 26.4 0.3821.34 Example 2 56.5 15.7 27.8 0.361 1.3 Example 3 57.2 15.9 26.9 0.3711.34

The absorption and photoluminescence spectra of the semiconductornanoparticles obtained in Examples 1 to 3 were measured. The results areshown in FIG. 2 (absorption spectrum, Example 1: solid line, Example 2:dot-and-dash line, Example 3: dotted line) and FIG. 3 (photoluminescencespectrum, Example 1: solid line, Example 2: dot-and-dash line, Example3: dotted line). The absorption spectrum was measured at wavelengths of190 nm to 1,100 nm using a diode array spectrophotometer (manufacturedby Agilent Technologies Japan, Ltd., trade name: Agilent 8453A). Thephotoluminescence spectrum was measured at an excitation wavelength of365 nm using a multi-channel spectrometer (manufactured by HAMAMATSUPHOTONICS K.K., trade name: PMA11). The wavelength and the full width athalf maximum of a sharp photoluminescence peak observed in thephotoluminescence spectrum in each Example are as follows:

Example 1: around 590 nm, full width at half maximum 42 nm

Example 2: around 590 nm, full width at half maximum 50 nm

Example 3: around 590 nm, full width at half maximum 55 nm

In measurement of the photoluminescence lifetime, using a fluorescencelifetime measurement device (trade name: Quantaurus-Tau) manufactured byHAMAMATSU PHOTONICS K.K, the semiconductor nanoparticles were irradiatedwith light having a wavelength of 470 nm as the excitation light to emitlight having a wavelength different from that of the excitation light,and a decay curve of the photoluminescence at around the peak wavelengthof the sharp photoluminescence peak was measured. The obtained decaycurve was divided into three components by parameter fitting using afluorescence lifetime measurement/analysis software U11487-01,manufactured by HAMAMATSU PHOTONICS K.K. As a result, τ₁, τ₂, and τ₃,and contribution rates of respective components (A₁, A₂, and A₃) weredetermined as shown in Table 3 below.

TABLE 3 Photolumines- τ₁ A₁ τ₂ A₂ τ₃ A₃ cence lifetime (ns) (%) (ns) (%)(ns) (%) (ns) Example 2 6.56 50.5 25.9 41.3 95.1 8.2 25.9

The photoluminescence lifetime (τ2: 25.9 ns) of the main component inExample 2 was similar to the lifetime (30 to 60 ns) of CdSenanoparticles in which band-edge emission was confirmed, and thisphotoluminescence was found to be due to the band edge.

In Example 1, the intensity of the band-edge emission was smaller thanthe intensity of other types of photoluminescence (including defectluminescence) observed at a longer-wavelength side than the band-edgeemission. It is assumed that this is because the temperature of the heattreatment in the first step in Example 1 was lower than that in Example2.

In the absorption spectra in Examples 2 and 3, an exciton peak wasobserved at around 550 nm.

Example 4, Comparative Examples 1 and 2

Silver acetate (AgOAc) and indium acetate (In(OAc)₃) were weighed sothat Ag/(Ag+In) is 0.3 (Comparative Example 1), 0.4 (Example 4), and 0.5(Comparative Example 2), and the total amount of two metallic salts is0.25 mmol. Silver acetate (AgOAc), indium acetate (In(OAc)₃), and 0.25mmol of thiourea were placed into a mixture of 0.10 cm³ of oleylamineand 2.90 cm³ of 1-dodecanethiol, and dispersed. The dispersion of silveracetate and indium acetate was put into a test tube together with astirring bar, and nitrogen substitution was performed, followed byheating at 150° C. for 10 minutes (heat treatment in the first step) andfurther heating at 250° C. for 10 minutes (heat treatment in the secondstep), while stirring the content in the test tube under a nitrogenatmosphere.

After the heat treatment, the obtained suspension was allowed to cool,followed by centrifugal separation (with a radius of 146 mm at 4,000 rpmfor 5 minutes).

In Comparative Example 1, after the precipitates were washed withmethanol, chloroform was added to the precipitates, followed bycentrifugal separation (with a radius of 146 mm at 4,000 rpm for 15minutes), and the supernatant solution was collected, and then thefollowing measurement was performed.

In Example 4 and Comparative Example 2, a supernatant solution was takenout. Then, methanol was added to this until nanoparticles wereprecipitated, followed by centrifugal separation (with a radius of 146mm at 4,000 rpm for 5 minutes), thereby precipitating the nanoparticles.The precipitates were taken out and dissolved in chloroform, and thefollowing measurement was performed.

Although the nanoparticles in Example 4 were produced under the sameconditions as in Example 2, they were produced separately from those inExample 2, and thus the average particle size and so on are slightlydifferent from those in Example 2.

The shape of the obtained semiconductor nanoparticles was observed, andthe average particle size was measured. The shape of the obtainedparticles was a spherical or polygonal shape. The average particle sizein Comparative Example 1 was 10.4 nm, the average particle size inExample 4 was 4.3 nm, and the average particle size in ComparativeExample 2 was 3.8 nm.

Next, the proportion of each of Ag, In and S atoms in the obtainedsemiconductor nanoparticles was determined using a fluorescent X-rayanalyzer (manufactured by RIGAKU Corporation, trade name: EDXL300), byassuming that the total number of atoms of Ag, In, and S included in thesemiconductor nanoparticles was 100. The result is shown in Table 4.

TABLE 4 S Ag In Ag/(Ag + In) S/(Ag + In) Comparative 55.5 19.3 25.2 0.431.24 Example 1 Example 4 57.3 15.9 26.9 0.37 1.34 Comparative 55.3 21.823 0.49 1.23 Example 2

The absorption and photoluminescence spectra of the semiconductornanoparticles obtained in Comparative Examples 1 to 2 and Example 4 weremeasured. The results are shown in FIG. 4 (absorption spectrum, Example4: solid line, Comparative Example 1: dotted line, Comparative Example2: dot-and-dash line) and FIG. 5 (photoluminescence spectrum, Example 4:solid line, Comparative Example 1: dotted line, Comparative Example 2:dot-and-dash line). Measurements of the absorption spectra and thephotoluminescence spectra were performed using the devices and methodsmentioned in Examples 1 to 3.

As shown in FIG. 5, in each of the photoluminescence spectra inComparative Examples 1 and 2, a small peak is observed at a wavelengthof around 580 nm, which corresponds to the peak wavelength of theband-edge emission in Example 4, but the intensity was extremely small.

An exciton peak was observed at around 570 nm in the absorption spectrumin Comparative Example 1, and an exciton peak was observed at around 530nm in the absorption spectrum in Comparative Example 2.

Example 5

<1> Production of Cores (Primary Semiconductor Nanoparticles)

Silver acetate (AgOAc) and indium acetate (In(OAc)₃) were weighed sothat Ag/(Ag+In) is 0.4, and the total amount of two metallic salts is0.25 mmol. Silver acetate (AgOAc), indium acetate (In(OAc)₃), and 0.25mmol of thiourea were placed into a mixture of 0.10 cm³ of oleylamineand 2.90 cm³ of 1-dodecanethiol, and dispersed. The dispersion of silveracetate and indium acetate was put into a test tube together with astirring bar, and nitrogen substitution was performed, followed byheating at 150° C. for 10 minutes (heat treatment in the first step) andfurther heating at 250° C. for 10 minutes (heat treatment in the secondstep), while stirring the content in the test tube under a nitrogenatmosphere.

After heat treatment, the obtained suspension was allowed to cool,followed by centrifugal separation (with a radius of 146 mm at 4,000 rpmfor 5 minutes), so that a supernatant solution was taken out. Then,methanol was added to the supernatant solution until nanoparticles wereprecipitated, followed by centrifugal separation (with a radius of 146mm at 4,000 rpm for 5 minutes), thereby precipitating the nanoparticles.Ethanol was added to the precipitates, and the solution was stirred,followed by centrifugal separation (with a radius of 146 mm at 4,000 rpmfor 5 minutes) again, thereby precipitating the nanoparticles. Theprecipitates were taken out and dissolved in chloroform, and thefollowing measurement was performed.

The shape of the obtained semiconductor nanoparticles was observed, andthe average particle size was measured. The shape of the obtainedparticles was a spherical or polygonal shape. The average particle sizewas 4.6 nm.

Next, the proportion of each of Ag, In and S atoms in the obtainedsemiconductor nanoparticles was determined using a fluorescent X-rayanalyzer (manufactured by RIGAKU Corporation, trade name: EDXL300), byassuming that the total number of atoms of Ag, In, and S included in thesemiconductor nanoparticles was 100. The result is shown in Table 5.

TABLE 5 S Ag In Ag/(Ag + In) S/(Ag + In) Example 5 56.8 15.8 27.4 0.371.31 (core)

The absorption and photoluminescence spectra of the obtainedsemiconductor nanoparticles were measured. The methods for measuring theabsorption spectrum and the photoluminescence spectrum are the same asin Examples 1 to 3. The result is shown in FIG. 6. As shown in FIG. 6,in the photoluminescence spectrum of the semiconductor nanoparticles, asharp photoluminescence peak having a peak wavelength at a wavelength ofaround 580 nm and a full width at half maximum of 50 nm was observed,and a broad photoluminescence peak having a peak wavelength at awavelength of around 705 nm was observed.

The photoluminescence lifetime was measured by the same method as thatemployed in Example 2. The result is shown in Table 6.

TABLE 6 Photolumines- τ₁ A₁ τ₂ A₂ τ₃ A₃ cence lifetime (ns) (%) (ns) (%)(ns) (%) (ns) Example 5 8.13 65.9 39.3 31.3 163 2.8 39.3 (core)

The photoluminescence lifetime (τ2: 39.3 ns) of the main component ofthe semiconductor nanoparticles was similar to the lifetime (30 to 60ns) of CdSe nanoparticles in which band-edge emission was confirmed, andthis photoluminescence was found to be due to the band edge.

<2> Production of Core-Shell Semiconductor Nanoparticles (Formation ofShell)

Using the semiconductor nanoparticles produced in the above section <1>as cores (primary semiconductor nanoparticles), a shell was formed onthe surface of the core.

More specifically, 1.0×10⁻⁵ mmol (10 nmol) of the primary semiconductornanoparticles produced in the section <1> at amount of nanoparticles,5.33×10⁻⁵ mmol of gallium acetylacetonate (Ga(acac)₃), 5.33×10⁻⁵ mmol ofthiourea, 2.9 cm³ of oleylamine, and 0.1 cm³ of 1-dodecanethiol were putinto a test tube and held at 300° C. for 60 minutes, and then theheating source was turned off to allow the solution to cool. Then, thesolution was subjected to centrifugal separation (with a radius of 146mm at 4,000 rpm for 5 minutes), thereby precipitating the nanoparticles.The supernatant solution was discarded, methanol was added to theprecipitates, and the solution was stirred, followed by centrifugalseparation (with a radius of 146 mm at 4,000 rpm for 5 minutes) again,thereby precipitating the nanoparticles. The precipitates were takenout, ethanol was added thereto, and the solution was stirred, followedby centrifugal separation (with a radius of 146 mm at 4,000 rpm for 5minutes) again, thereby precipitating the nanoparticles. Theprecipitates were taken out and dissolved in chloroform, and thefollowing measurement was performed.

The shape of the obtained core-shell semiconductor nanoparticles wasobserved, and the average particle size was measured. The shape of theobtained particles was a spherical or polygonal shape. The averageparticle size was 6.5 nm. The average thickness of the shell was about0.95 nm based on a difference in the average particle size between thecores and the core-shell semiconductor nanoparticles.

The absorption and photoluminescence spectra of the obtained core-shellsemiconductor nanoparticles were measured. The methods for measuring theabsorption spectrum and the photoluminescence spectrum are the same asin Examples 1 to 3. The result is shown in FIG. 7. As shown in FIG. 7,in the photoluminescence spectrum of the semiconductor nanoparticles, asharp photoluminescence peak (band-edge emission) having a peakwavelength of around 580 nm and a full width at half maximum of 49 nmwas observed. The intensity of a broad photoluminescence peak (defectluminescence) having a peak wavelength at a wavelength of around 710 nmwas extremely small. The intensity of the band-edge emission that isnormalized by the intensity of the defect luminescence was 18. Thisresult showed that covering with a specific shell is effective forincreasing the proportion of the band-edge emission.

The photoluminescence lifetime was measured by the same method as thatemployed in Example 2. The result is shown in Table 7.

TABLE 7 Photolumines- τ₁ A₁ τ₂ A₂ τ₃ A₃ cence lifetime (ns) (%) (ns) (%)(ns) (%) (ns) Example 5 5.44 68.2 51.9 25.7 248 6.1 248 (core)

The photoluminescence lifetime (τ3: 248 ns) of the main component of thecore-shell semiconductor nanoparticles was confirmed to be longer thanthat of the core. A component (τ2, A2) with a photoluminescence lifetimeof 51.9 ns was observed with their intensity being similar to that ofthe main component. This photoluminescence lifetime was similar to thefluorescence lifetime (30 ns to 60 ns) of the component with the largestcontribution rate in the fluorescence emitted by CdSe (nanoparticles) inwhich band-edge emission was confirmed.

The obtained core-shell semiconductor nanoparticles were observed withHAADF-STEM (manufactured by JEOL Ltd., trade name: JEM-ARM200F Cold).FIG. 8 shows an HAADF image. In the HAADF image, a crystal core with aregular pattern and a shell surrounding the core and having no regularpattern were observed, and the shell was observed to be amorphous in theparticles.

For comparison, an HAADF image of the semiconductor nanoparticles(cores) obtained in the section <1> is shown in FIG. 9. In the HAADFimage shown in FIG. 9, only crystal particles with a regular patternwere observed, and a region having no regular pattern surrounding themwas not observed.

In the obtained core-shell semiconductor nanoparticles, the atomicpercentages of S (sulfur) and Ga (gallium), which were included in acentral region in which a crystal core with a regular pattern wasobserved in the HAADF image, and in a surrounding region in which ashell with no regular pattern was observed in the HAADF image, wereanalyzed with an energy dispersive X-ray spectrometer (EDS)(manufactured by HORIBA, Ltd., trade name: EMAX Energy). The atomicpercentages of S and Ga are proportions assuming that the total numberof atoms of Ag, In, Ga, and S is 100%. The result of the analysis isshown in Table 8.

TABLE 8 S Ga Central region 58.1 4.6 (core) Surrounding region 74.7 15.7(shell)

As shown in Table 8, the proportions of Ga and S in the surroundingregion were higher than those in the central region. This resultdemonstrated that, in terms of element composition, the obtainedparticles have a core-shell structure in which GaS_(x) is segregated onthe surfaces of the particles.

The embodiments of the present disclosure provide the semiconductornanoparticles that enable the band-edge emission and can be used as awavelength conversion material of a light-emitting device or as abiomolecule marker.

The invention claimed is:
 1. A method of producing semiconductornanoparticles, the method comprising: (a) providing a salt of Ag, a saltof In, a source compound of S, and a first organic solvent; and (b)placing the salt of Ag, the salt of In, and the source compound of Sinto the first organic solvent so that a ratio of a number of atoms ofAg to a total number of atoms of Ag and In is 0.33 or more and 0.42 orless to obtain semiconductor nanoparticles.
 2. The method of producingsemiconductor nanoparticles according to claim 1, further comprising:producing the semiconductor nanoparticles by heating the first organicsolvent that contains the salt of Ag, the salt of In, and the sourcecompound of S at a temperature of 230° C. or more and 260° C. or lessfor 3 minutes or more in step (b).
 3. The method of producingsemiconductor nanoparticles according to claim 1, further comprising:producing the semiconductor nanoparticles by heating the first organicsolvent that contains the salt of Ag, the salt of In, and the sourcecompound of S at a temperature of 30° C. or more and 190° C. or less for1 minute or more and 15 minutes or less, and then at a temperature of230° C. or more and 260° C. or less for 3 minutes or more in step (b).4. The method of producing semiconductor nanoparticles according toclaim 1, wherein the source compound of S is placed into the firstorganic solvent so that a ratio of a number of atoms of S to the totalnumber of atoms of Ag and In is 0.95 or more and 1.20 or less in step(b).
 5. The method of producing semiconductor nanoparticles according toclaim 1, wherein a ratio of a number of atoms of Ag to the total numberof atoms of Ag and In in the semiconductor nanoparticles produced issmaller than the ratio of the number of atoms of Ag to the total numberof atoms of Ag and In, in the salt of Ag and the salt of In that areplaced into the first organic solvent.
 6. The method of producingsemiconductor nanoparticles according to claim 1, wherein the firstorganic solvent is a mixed solvent that contains at least one solventselected from thiols comprising a hydrocarbon group with a carbon numberof 4 to 20 and at least one solvent selected from amines comprising ahydrocarbon group with a carbon number of 4 to
 20. 7. The method ofproducing semiconductor nanoparticles according to claim 1, furthercomprising: (c) after step (b), performing a centrifugal separation bycentrifuging the first organic solvent that contains the semiconductornanoparticles produced, and obtaining a supernatant solution after thecentrifugal separation; and (d) adding a second organic solvent to thesupernatant solution, followed by centrifugal separation, and taking outthe semiconductor nanoparticles as precipitates.
 8. A method ofproducing core-shell semiconductor nanoparticles, comprising: providinga dispersion in which the semiconductor nanoparticles that are producedby the method according to claim 1 are dispersed into a solvent; andadding, to the dispersion, a compound containing a Group 13 element andan elemental substance of a Group 16 element or a compound containingthe Group 16 element, to form a semiconductor layer consistingessentially of the Group 13 element and the Group 16 element on asurface of each of the semiconductor nanoparticles.
 9. The method ofproducing core-shell semiconductor nanoparticles according to claim 8,wherein the Group 13 element contains Ga and the Group 16 elementcontains S.